Reverse seroconversion of hepatitis B virus (HBV) after allogeneic BMT is rare. We present a case of HBV reactivation late after allogeneic BMT which responded well to lamivudine therapy. A 35-year-old woman with CML received an allogeneic BMT. Before BMT, the patient had immunity to HBV, with serum antibodies against hepatitis B surface antigen (HBsAb), and the donor was completely negative for HBV. Four years after BMT, acute hepatitis occurred with a detectable level of HBV-DNA. Lamivudine rapidly reduced transaminase and bilirubin levels, and serum HBV-DNA decreased to negative. Retrospective analysis revealed that there had been a gradual decrease in serum HBsAb titers after BMT. Administration of lamivudine immediately after HBV replication may be more effective than vaccination of hepatitis B surface antigen-negative donors before BMT.